In the related art, fluorescence detection systems using fluorescent pigments as labeled substances have been widely used in the fields of biochemistry and molecular biology. The fluorescence detection systems can be used to perform, for example, gene arrangement, analysis of gene mutation and polymorphism, and evaluation of protein separation and identification and are thus used to develop drugs and the like.
As an evaluation method using fluorescent labeling, as described above, a method of distributing biological compounds such as proteins in gels through electrophoresis and acquiring the distributions of the biological compounds through fluorescence detection is well used. In the electrophoresis, an electric field gradient is generated in a solution such as a buffer solution by putting an electrode in the solution and causing a direct current to flow. At this time, when protein, Deoxyribonucleic acid (DNA), or ribonucleic acid (RNA) with a charge is present in the solution, molecules with a positive charge can be attracted to an anode and molecules with a negative charge can be attracted to a cathode. Thus, separation of the biological molecules can be performed.
Two-dimensional electrophoresis which is one of the evaluation methods using the foregoing electrophoresis is an evaluation method in which biomolecules are distributed in a gel two-dimensionally by combining two types of electrophoresis methods, and is considered to be the most effective method available to perform proteome analysis.
As a combination of the two types of electrophoresis methods, a combination of “isoelectric point electrophoresis using a difference in an isoelectric point of the individual protein” which is the first dimension and “sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) performing separation with the molecular weight of the protein” which is the second dimension is generally used. Fluorescence pigments are added to proteins, which are biomolecules separated in this way, before electrophoresis is performed or after electrophoresis is performed.
Image reading devices that emit excitation light to a gel support in which biomolecules (proteins) are distributed two-dimensionally, the gel support being produced in the above manner, acquire generated fluorescence intensities, and display fluorescence distribution (protein distribution) images based on the fluorescence intensities are widely used in the fields of biochemistry and molecular biology.
As a method of maintaining a two-dimensional distribution of the biomolecules, a method of separating proteins in the gel and subsequently transferring the proteins from the gel to a membrane using electrophoresis or capillarity as well as maintaining the distribution of the biomolecules in the gel can be performed. In this case, as in the case of image reading performed using the gel support, the fluorescence distribution of a transfer support which is the membrane can be imaged by an image reading device.
As a first fluorescence detection device of the related art used in an image reading device that reads a biomolecule distribution image from a gel support or a transfer support in which the biomolecules are distributed two-dimensionally, as described above, there is an optical head scanning type device that includes an optical head that irradiates a sample with excitation light and that guides fluorescence generated from an excitation light emission unit to detection means and scanning means for moving the optical head over the sample at a constant speed (for example, see Japanese Unexamined Patent Application Publication No. 10-3134 (PTL 1)).
In the first fluorescence detection device of the related art, excitation light having a narrow beam emitted from a light source is reflected in a sample direction by a mirror, subsequently passes through a hole formed in a part of a mirror, and is collected at one point on the sample by a lens so that the sample is excited. At this time, the fluorescence generated from the sample follows the same optical path as the excitation light, is converted into parallel light by a lens, is subsequently reflected from a mirror surface with a hole, passes through a laser light cut filter, and is then detected.
In the first fluorescence detection device of the related art, among the fluorescence generated from the sample, a component of light that has passed through the hole is not guided to a photodetection element. However, since the emission angle of the fluorescence is wide, the diameter of the excitation beam becomes thick and most of the fluorescence emitted from the sample is guided to the photodetection element. The fluorescence intensity from each emission point is acquired by scanning the optical head over the sample two-dimensionally and a fluorescent image is generated based on the information regarding the fluorescence intensity.
As a second fluorescence detection device of the related art, there is a fluorescence detection device improving sensitivity more than the first fluorescence detection device described above (for example, see Japanese Unexamined Patent Application Publication No. 2000-162126 (PTL 2)). In the fluorescence detection device, a reflection optical system is disposed on a side of a sample opposite to a side on which an optical head is disposed in order to also detect fluorescence emitted from the opposite surface side, and thus the amount of fluorescence detected increases.